Carrier-wave transmission and communicating system



Jan. 5 1926.

E. H. SMYTHE CARRIER WAVE TRANSMISSXON AND commumcmme SYSTEM FiledMay's, 1921 4 Sheets-Sheet l Jan. 5 1926. 1,563,194 E. H. SMYTHE CARRIERWAVE TRANSMISSION AND COMMUNICATING SYSTEM Filed May 5, 1921 4Sheets-Sheet 2 TJHJUL JUMLW I i 1 q Jan. 5 1926.

E. H. SMYTHE CARRIER WAVE TRANSMISSION AND COMMUNICATING SYSTEM FiledMay-5, 1921 Sheets Sheei Lian. s ,1926. 1,568,194

E. H- $MYTHE CARRIER WAVE TRANSMISSION AND COMMUNICATING SYSTEM FiledMay 5, 1921 4 Sheets-Sheet 1 I w b L R K {M mm P F K) E V R I I g.\ I.

//1\/@ x7 for: Edw/n f7. Gmyfhe Patented Jeni. 5, i926.

"E" P AT- N nnwxn n. sMYTnE. on nvenswon, ILLINOIS. Assxonon: TO WESTERN'ELnc'mIo COMPANY, INCORPORATED, on new Yonx, in; Y., A conPonA'rIoNoFnew YORK.

' seamen-wave mnansmssron "im lication filed May 5,

f0 all whom it may concern:

Be it known thatI, Enwm H. SMYTHEf a citizen of the United States,residing: at Evanst'on. in the couutyof Cookand State of Illinois. haveinvented certain new and useful liiiproveuieuts 1n Carrier-Wave 7Transmission and Cou'imunicating Systems,

of whieh the following is a full, clear,-concise. and exact description.7 ,7

This invention relates to an improved communicating system, and moreparticle larly to a conu'i'iunicating system of the typeiu-which carrierwaves or oscillations, either free 'orgiuded, are employed as. the

medium for conveying energy for efle-cting the operation and control ofelectric signal ling, controlling and comnnmicating ap- 1 paraiuslocated atseparated points in the system.

The invent on especially relates to a system n which so-called carriercurrents, in

the form ofhigh frequency waves guided upon wires, are used as themedium of electrical connection between the electrical devices andapparatuses located at the separated points in the system. These carriercurrents are assigned such frequencies that they are non-interfering onewith another. so that each carrier serves as a separate and distinctconnecting medium superposed upon the line or conductor over which thecarrier is transmitted. Each 'or' the carriers is modificrl or modulatedin accordance with the voice currents or other signalling or controllingcurrents that are to he carried. The carriers thus modulated aretransmittcdover the toll or long distance line and are passed throughindividual receiving" arrangements each of which is selective of onecarrier frequency to the exclusion-of the others, and each of whichserves to reproduce from the modulated carrier-the voice or signalcurrents with which the ear The apparatus which is used to transmit andreceive by means of any one high frequency wave as distinguished fromthat which is used fox-transmitting and receiving by means of a wave ofadiflerent frequency, is con'unonly knownas a channel, or a highfrequency communication channel. I

It is the object of the invention to provide a system in which afrequency selective communication channel associated with a line is AnncoMMUnIoATINo SYSTEM; 1921 semi in. 467,643.

frequencyselective channel. whenin use'is guardedt against intrusion; inwhich any one of a "plufality of frequency selective channels is made'availablefor inter-communication oetwe'en any twoof awplurality whichthe-high frequency, channels are associated; ina which means areprovided for enabling any one of the stations simultaneouslyto employfor use/all of the'ch'am nels,' or as manyas are not in use forcomstations connected with the line with munication between otherstations; in'which each; station has means. for applying to the line {aplurality of dilierin'g frequency waves to eachone of which a diiierentdistant station of the line is'responsive, so that each station mayselect any other station on the line; in which the selection of acalledstation bye calling station is accompanied 'by the production or'displayof a signal; in which ineans are provided in the initiation ot-a callfor automatically testing the frequency selective communicatingehannels; appropriating one thatis not in use; in which the automaticchannel testing andselecting operation is an automatic switch that mayhe located at the called station and maybe made operativehythe select onof that station; in which a special calling high or other frequencychannel is employed for effecting the selection of a particular calledstation by a calling station; in which the selection of the difl'erentstations through the medium of the special-calling channel isaccomplished hy' the 'lnodulation ot the carrier frequency of. thechannel with difierent lowerfrequeucies to each one of which a differentone of-the stations is selectively responsive; in whichtheselec'tion ofan idle communicating channel by'the automatic testingand selectingswitch at the called station is accompanied loyi -the identifica tion ofthe selected communicating channel at the calling station; in which thecomple tion of connection with the identified channel at the callingstation causes the identification of the said channel at the calledstation;-in which the completion ofconnection.

with the identified channel at the called stetion or failure to select"a channelcauses the restoration of the channel 1 testing "andseperfornied by a that station is returned.

of the invention in Eli tion or upon -tioned above,

other stations.

looting switch; in which a second selection of the called station isprevented until the automatic testing and selecting a )paratus of tonorma in which all other stations are prevented from using the specialcalling channel while it is in use by any one of them; in which thecalling channel is automatically made available for the use of otherstations upon the completion of the station and channel seleciailure toselect a channel; and in ivhich. the condition of use or disuse of aselected communicating channel at each of the two stations that areusing. it is automatically indicated at the other station.

The foregoing are some of the principal objects that the system of thepresent in; vention attains. The foregoing and other features are allincluded in the present embodiment of the invention, but are capableofseparate use where it may be desirable to include only part of thefeatures that are herein disclosed. In addition, the system thepreferred form in which it is disclosed. embodies a'munber of otherfeatures incidental to those menand incidental to its use as part of acomprehensive system providing for inter-communication between toll or IThe manner in which the above mentioned objects and features areattained and realized will be explained in the following descriptionotthe preferred form of the invention,- and the structures andcombinations through which the objects are attained will be moreparticularly defined in the appended claims.

The preferred embodiment of the invention is illustrated in theaccompanying drawings. in which Figs. 1 to 4 inclusive, arranged asindicated in 5, show diagrammatically as much of the circuitorganization at one roll station of the system as is necessary to enablethe invention to be understood. and Fig. 6 shows an alternative detailcircuit. Tit-will be understood. without the necessity of fulldiagrammatic representation, that there are al plurality of stationsconnected with the toll line L, a total of four such stations beingassumed in the following description to be present, andqthat each ofthese stations may be in all respects identical in apparatus andvcircuit arrangement with the particular station illustrated in Figs. 1to 4;,the only differences between the stations being in the use oi aharmonic producer at one of the stations and or" a harmonic reproducerat each of the other stations, and in the particular calling channelmodulating frequencies whiclfcach station is adapted to trans mit and towhich it is adapted to respond, as will be fully explained hereinafter.

Referring now to the preferred circuit organization as illustrated inFigs. 1 to t of? the drawing, only as much of the Wiring at a single oneof the stations of the toll line is illustrated as is necessary toenable the invention to be understood; it being possible for one skilledin the art to which this invention pertains. by mere duplication andextension ol the wiring and apparatus illustrated. and without theexercise of inventiou. to organize a system of such capacity as may bedesired. .\s illustrated iu the circuit diagram of Figs. 1 to l. eachone of the toll'stalions or toll points is couucctcd with the toll lineL through the medium ol a twoway repeating coil or conjugate coil 100.this coil terminating in an arlilicial line or network N that isconstructed and arranged as is well. understood in the art. to balanceand match in its'charactcristics the toll line with which connection ismade. Extending from the windingsof this twoway repeating coil arc twopairs of conductors, T and ll. the pair 'T serving as the transmittingpath to the toll line of the locally generated currents. and the pair Rserving as the receiving path from the toll line of the currentsgenerated at and transmitted from the other stations of the line. Thetransmitting and receiving paths each have connection wi h the localapparatu oi the four communicating channels (1 C. C and C" and with thelocal apparatus oi the calling channel .\t each ofthc toll stationsthere is also a connection oi either the transmitting. path T or thereceiving path Rwith the base frequency channel BC The connection of thetransmitting path T is with the transmitting branches TC, TBC. TC TC T iand TC of the various chan nels respectively: and the connection of thereceiving path R at each station is with the receiving branches RC. RFC.RC. RC. RC and RC ot' the various channels rcspcctively. Thetransmitting branch apparatus of the base frequency (lumnvl BC isprovided at only one of the toll stations. At the other three stationsthe transmitting branch apparatus of this channel is omitted and theapparatus indicated in comicc' tion with the receiving branch Rilfi isprovidedin its stead. At the station where the transmitting branchapparatus of the, base frequency channel, BC is provided. the circuit isclosed at the points X and is opened at the points X At the otherstations where the receiving branch apparatus of the base frequencychannel is used. the circuits are closed at the points X and are openedat the points X. Obviously, in each case the opening of the circuits atlbc points X or X ,.as the case may be, in omitting one or the other ofthe groups of apparatus, is accompanied by a closing of the transmittingpath pa'h B at the points X to make these paths continuous for thepassage o'fcurreut to and T at the points X or the receiving branch TCof n) leg carrier frequency lower niodulliting from the other channelbranches associeted with these paths. I

The organization of the transmitting and receivingbranches of thevarious channels 5 will be specifically described hereinafter. As ageneral description of-the organize t'ieny it may he suid that thetransmitting ganized as to apply to the toll line the call quency by theV 7 keys KB, KC and Kl) respectwely Each of these keys in heme operatedcauses the application to the calling carrier frequency of theparticular adapted to bring about the response of thecreceivingapparatus oi the particulzir one of the distant toll stationsthat, lS llQlllQ called.

. The three station selecting keys KB, KC and KD have associated iviththem a pilot ongiierd lamp 79that is illuminated to in form the operatorthat the selecting apparutus is in use, either locally or at a distantstation.

The receiving hranch'RC of the calling channel is so organizedthat theunmodulisted component of the calling carrier frequency received fromthe toll line brings shout th operetion'ot the relay 66, while themodulated component, if the modulating frequency is that to ivhich thisparticular station is adapted to respond, brings about i the operationof the relay 67. The relay 66 acts through the medium of the relay 58 torender ineffective the station selecting keys of? the comn'umicntingchannel transmitting uuton'uitic selection of the ealled toll station ofad .itlle onelof the communicating channels.

in the oscillator OP. and its harmonics are trihuting circuit comprisingthe conductors 47, 48,'fro1n which separated out and applied ting path Tbyway of the branch TBG, I while the harmonies are selectively passed tothe transmitting apparatus of the comthe calling channel C lSvSOorsupplied by theeoseih tortion of a base frequency SllpPllOQbY the.

Each of the niodulating MC and MD is macleopera 9 tire to apply its-particular modulating fife-y pressing of corresponding;

1mg conductors- 85 l'1'l()( ulatiugfrequency Kill. KC and KD, end tolight the key guard E lziuip 79 to maintain the existing condition pr gi ksL apparatus. The relay (l7 acts through the nu-diiun of the relay 53to bring about the c inform the calling tolloperator The Apparatus inthe transmitting branch THC of the base frequency channel BC is l 4 soorganized as to distort in the harmonic producer HPH e louse frequencygenerated The base frequency conducted to a disthe base frequency" is tothe transmitto notify theoperator at th n unicating channels C C (PjandG to supply the earrier frequencies for conveying the speech andsignalling modulations over the respective channels.

The apparatus in thejroceiving branch REC of the base freqi iencychannel BC is so orgenized lthat the base frequency received fromtheparticular one of the four toll stations herelthehase frequencyproducing a'pparatusis located, is separated, out from the other-carrierfrequencies passing over the receiving path R, is amplified, passedthrough a harmonic reproducer HRP, and the harmonics of the basefrequencyfthus produced are supplied to the distributing conductors4T,;4t8,to supply the carrier frequencies required by p the eo'mmumcatmgchannels C, C C and G.

The organization of each of the eommunicatingchannelsfCl, C C and C issuch that th voice currents received from the local telephone circuit byway of the talkand 86ers passed'through the two-Weylrepeating coil- 33,caused to modulate the carrier frequency received from'the distributingconductors 47, 48, and the modulated-carrier frequency is" conductedtothe transmitting pat-l1 T. The

organization of eachof -the communicatingchannelsis alsolsuchthat thecharacteristic carrier frequency to whiehzthechannel is adapted torespond is separated out from the e quency components are'conveyed byway of the two-way repeating coil 33 to the local other frequenciespassingfover the receiv- 19 acts through the medium of the relays l6 and'10 to control'the signallingend channel selecting operations. I Eachcommunicating channel w th its lowjfrequency extension constitutes a,trunk for estdblishing communication withanother' station."

nch of theeommunica-tin "ehannels'C,

path R, is detected, and the voicefretalking conductors '85- and 86,While the carrier frequency components are divert-ed and caused tooperatethe'relay 19. The relay C; C 'and'C is terminate locally upon I33nd4'res"peotively. 'With these four spr ng jacks are essociated theassigning lamps 11, 21; 31 11nd 41 and the canin lamps 12; 22, 32 andltQ, respectively. The- Emctionof the assigning lamp isfto of theparticularchannel that has been selected for. use; and the function ofis to inform the operator the called toll station of the receipt of a,call upon the corresponding channel spring the spring 'jeckterminals ofthe communi- I I jack, Asso elated Wlllll the channelterininals andtheir corresponding assigningand calling lamps,- there is acall pilotlamp 27 the calling lano'pl,

cating channels the connections are extended .to spring 130k terminals77 of local stations SS by means of a plurality of connecting.

cord circuits 70. of which there may be enough provided to utilize allof the chair nels simultaneously. or as many of them as are not at thetime in use between other stations. Obviously, connection between thecommunicating channels and the local lines may be cll'e cted in anyother desired way of the many that are well-known in the art; as forinstance, by means of circuits involvin the use of automatic switches.

he selection of an available couu'nunicau ing channel, in the particularembodiment of the invention illustrated. is cilected by means of anautomatic switch S that is set into operation by the energization of therelay 5 3 under the control ot the relay 6? which is responsive to thedetected modulating component of the calling carrier frequency. Themovement of this switch in its channel testing and selecting operationand the lighting of the lamp signals associated with the channel that isselected for use are under the control of lays 10, 13.

group re 14, 15 and 16 associated with the channel C and like groups ofrelays associated with the other communicating cnannels. As-

sociated with the channel selecting switch 5'3 are a stepping magnet 60.a release magnet 62, and the switch controlling relays 54c and 56. Thechannel selecting switch S with its associated magnets and relays, asWell as the relays 50, 51, 52, 55, 57, 58 and 59 that cooperate in thecontrol of the channels, are common to all of the channels of the tollstation. I

The organization and operation of that part of the system which acts togenerate, modulate, transmit, receive and de-rnoclulate or detect thehigh frequency carrier currents employed in the systeiuis as follows:The frequency of the carrier waves selectively utilized by thecommunicating channels is determined, in the present embodiment of theinvention, by frequency generated by .a thermionic or vacuum tubeoscillator .0? located at one of the toll'stations or at some point uponthe toll line. This oscillator may be of usual and Well known type, suchas the aiulion type, with a teed-back connection from its output intoits input circuit. The frequency of the oscillatory current produced maybe regulated by an adustable piondenser, and may conveniently epsay,

ve oscillator OP supplies subst-antiall sinusoidal oscillations to theinput circuit of the harmonic producer HPH. An inductance 101 is bridgedacross this circuit. he magnitude of the oscillations supplied is suchas to overload the thermionic tube a base or control thousandoscillations per second. The

asagna H'PH; that is, to supply an input wave of greater amplitude thanthe tube can repeatwithout rlistortion; The repeated osci lla- ,tionsinthe output circuitot the tube HPH are theretm-c of non-sinusoidal'wavcform. which is the equivalent of base frequency current of sinusoidalform and a series of currents of frequencies which are multiples orharmonics of the base frequency. The plate current supply of theoverloaded tube or harmonic producer HPH is through a circuit comprisinga resistance 102 and an inductance 103 which constitutes an impedancethat is large as compared with the impedance between the plate andfilament of the tube. This impedance relationship assists in producingdistortion and in stabilizing the output circuit. put circuit of theharmonic producer HPH is connected with a distributing circuit 47, 48,by means of a transformer 104, and tho distributing circuit is thussupplied with both the'base andthe harmonic frequency oscillations.Fromthe distributing circuit 47, 48, the base and harmonic frequenciesare selected by means of tuned circuits'lead ing to the transi'nittingbranches of the various communicating: channels. and the transmittingbranch of the base frequency channel. The inductance of the transformer105 of the base frequency trans" mitting branch and the capacities ofthe variable condenser associated with it are so adjusted that only, thebase frequency oscillations are selected. Similarly the constants of thetransformers and condensers of the other tuned circuits leading from thedistributing circuit are so adjusted that The out each of these tunedcircuits selects a. ditterentone of the harmonics of the basefrequency?" The harmonic frequencies selected by the transmittingbranches of the communicating channels C "J", and C may be tentl1ousand,'fifteen thousand. twenty thousand, and twenty five thousandrespectively.

The base frequency oscillation selected from the distributing circuit bythe tuned circuit including" the transformer passes through apotentiometerllOG, from which it is supplied at the desired potential tothe input circuit of the amplifier HA. The output circuit (it theamplifier HA is inductively coupled with. the terminals of a band filterIlF through which the amplified base frequency oscillation passes to thetransmitting path T. The band filter HF is of the well-known typedescribed in Patent 1,227.113 to Camp ell. patented May 22, 19W. Thecharacteristics at this filter, which consists of similar sectionshaving series and shunt capacities and 'inductances is such that ittransmits oscillations of the base frequency with little attenuation,and

' 2 .tive channels.

--lectedis amplified by the amplifienHRAi,

y efiecuvely diverts from this branch currents.

ofh'equencies differing more thana slight amountfrom the base frequency.a

As has been stated, the base frequency 05-. I

i cillator. ;is provided at blltm one? station or point on the tollline. At the'other toll stations-or points the base or controllingfrequency thus supplied to the line passes channel BBC is provided witha tra'nsform- I "er 107 withassociated condensers to 'consti time atuned circuit-that acts to divert the J lfl' base "frequencyoscillationzfrom the receiving path to, the amplifier HRA by way of vthepotentiometer 108, while permitting the free passage of, the othercarrier frequencies through the receiving; path to "The base frequencythus seand the amplifier oscillations areimpressed 'upon theinput-circuit of the thermionic tube HRR' The magnitn'deof theoscillations aresuch asto overload this tube, with the result that, asin the case of the tube HPH associated 'with the base frequency; I

extendlng to the spring jack terminals of oscillator OP; the repeatedoscillations are l distorted into'a series of currents the fre-:quencies ofwhich-arethe harmonics of-the base it'reque'ncycurrent, thetube HRP thus, acting as a harmonic i-egen'erator. The base freguencyand its harmonics are then arm 3 pli ed by theainplificr HRAfand areimpressed upon the distributing circuit -17 48 throggh lthetransformer109. From the distributingcircuit the harmonic frequencies' of the baseor controlling oscillation are selectively su plied'to the transmittizg'"tnanche'sof the c anne1s.C -C ,"C ..and

way of'the tuned circuits-ifprev iously-irestation Where the basefrequency os'cillator'jOP. and the; harmonic producer HPH areF-located-the" *ferredto. Thus atfthe toll communicating channel carrierfrequencies are supplied d1rectly',"whil e"-at the other sta- .tionsthey are suppiiedjfby a harmonic regenerator HRP controlled by thebasehe l1ne.. Consequently the correspondingcarline. The organizationillustrated and described for producing a plurality ofharmonic carn'ierfrequencies; from a, base frequencyby distorting that frequency, and.for insuringidentical frequencies 10f the carrier oscillationsemployedat each of the various quencies at all points from a, singlebase frequency, is substantially in accordance with theOigfilliZfliZiQllwdiSOlOSGd in United Statespatentto Kendall No.1,446,572, Feb- 1mm 12.7,. c1223, land. British aten No.

I invention.

the station into the receivinpath R. The

their' 'respec- ,85 and .86 are consequently pressed at the properpotential upon the inquency oscillations transmitted over the toll tollstations by controlling" the carrier fre- 133.,426, respectively. Themethod and organization described above for synchronizingcorrespondingcarrier waves at all stations is the preferred Way of securing thisresult for use in connection with the present But: it will be understoodthat other ways of securing substantiallyidentical frequencies of thecarrier Waves at the difi'erent points at which they are applied may beused.

At each of the communicating channels 5 0 C and (3* the particularcarrier frequency diverted from-the distributing circuit 4.7, i8 byineansof the. corresponding tuned circuitisapplied to the terminals of apotentiometer 110 from which the carrier oscillationsat the desiredpotentialare applied to thein put circuit of an amplifier HAG. Theoutput circuit of the amplifier is connected by Wayof the potentiometer111 with the input circuit of'a modulator M0 The input circuit of themodulator MC also has connection byway of the inductive coupling 112 jand the potentiometer 113 with the appropriate winding of thet'wowayrepeating coil 33 in the low frequency or talking circuit. The talkingconductors and relay contacts with a balancing netv.work, thecharacteristics of which are made to match the characteristics of thelow frequency, lines adopted to be connected tvitli' the channel. Thevoice-currents. that reach the repeater 33 by rayof the conductorsrepeated and imput circuit of the modulator M6 and'are thcre combinedwith the carrier frequency derived from the distributingcircuit 47, 48

to cause correspondingly modulated current ,to 'fiow inthe outputcircuit of the module tor. The outputcircuitoi the .nrodulator isconnected through an inductive coupling 114 with a hand filter MEOrofthe type dis closed in the Oainphell patent previously referred to; Thecharacteristics of thisparticularform of theCainpbell band filter aresuch that it suppressesany currents of the frequency of the voicecurrents inthe talk= {ingco'nductors ofthe channel whichmight berepeated as low frequency currents by 1 the modulator. ,Thecorrespondmgband filter ofeach of the channels serves to transmit currents of arange of frequencies inlow frequency ortalkin circuit. i

' 'rhe band filters Mltof n r ery? frequencies extending. up and down ofthe other channe s "of the base Frequency channel municating channels,the band filter HF of the base frequency" channel BC, and the bandfilter MFG of the calling frequency channel C are allarranged seriallyin the transmitting path 13. 14: and the constants of these variousfilters are so adjusted that each filter passes freely to thetransn'iitting path the carrier frequency from its own channel while atthe same time preventing the frequencies supplied through the filterstrom being diverts into its ehanneh The hand filters DEC of thereceivingbranches of the various Iconiinunicating channels C. C, G aud Cthe tuned circuit ot' the receiving branch BBC BC, and the band filterDEC of the receiving branch RC of the calling channel C, are similarlyincluded serially in the receiving path 'H extending from the. t=:-:o-wav repeating coil 100; and the constants of the filters and the tunedcircuit are so adjusted that each diverts from the receiving path Itinto its own channel the particular frequency to which the channelapparatus is adapted to be responsive, while perr 'itting to pass freclythrough the series circuit the frequencies for which the other channelsare adjusted.

The receiving branch of each of the Conn inunicating channels isprovided with a band filter DFG adjusted, as stated above, to divert?into the channel the characteristic frequency band of that channel. Theoscillatorycurrent 'thus diverted is supplied through a potentituneter115 to the low side of a shielded transformer 116, the high side ofwhich is included in the input circuit oi. the detector DC. The Cpotential for the grid of this detector tube is derived from thefilament battery by including a resistance 117 in the filament currentsupply cir" cuit beyond the point where the input circuit of thedetector is connected with the filament circuit. The output circuit ofthe detector DC is connected to the input circuit. of the amplifier ACthrough the me drum of a transformer 118 which is designed to transmiteliicicntly the voice frequencies obtained t'rom the demodulation of themodulated carrier wave. as well as the umnodulated carrier component.The output circuit of the amplifier Ao is connected to the low passfilter LPG. designed to pass the. voice frequencies and exclude thecarrier frequencies, by way of a. transformer 119. The other terminalsof the low pass filter are connected to the l)1l( ge terminals ot' thetwoavay repeating coil 33, where the voice currents are divided betweenthe talking conductors SC) a d S and the balancing network N in such ain; as to have their eil'ects neutralized in the coil oi the' repeaterthat is cminected with the outgoing transmission branch of" the channel.The output circuit of the amplifier AC also has a branch pathing path Rof each uncut-v t'iscillator OP. and may be su through a high passfilter HPC which acts to pass the unmodulated carrier wave component andto prevent the passage of the de-inodulated voice frequencies. The otherteri-ninals of the high pass filter HPC are connected with the low sideof a transformer l'ltLthe high side ot which is connected in the inputcircuit. of the rectifier tube HRC The output of the rectifier isconnected with the winding of a relay 19 which is adjusted to res-pondand hold its contacts closed as long as the associated communicatingchannel is receiving the carrier frequency which it is adapted to divertfrom the receiving path R. The filament and plate energizing currentsare supplied to the apparatus of the receiving branch of the channelunder the control of a relay 1?, and arefsnpplied to the transmittingbranch of t'liei channel under the control of a relay 1S. llie'snpplcircuits are so arranged that'the filament and plate current is normallydisconnected. and is applied to the receiving and transmitting branchapparatus only when the correspomling relays grlzed.

17 and 1S enerhas previouslv been stated, the receivtoll station isarranged to receive a calling carrier frequency by way oi? thetransmitting branch TC of the calling chlinnel the calling carrier beinginodulated by lower frequencies in accordance with which o i themodulating branches lii B, MC and MD is made operative. In accord uncewith the present embodiment of the invention, the "ailing carrierfrequency is supplied by an oscillator DC the output circuit or which isconnected by way of potentiometer 121 with the input circuit of themodulator MC. 'lhecalling carrier may be of any suitable frequency thatditlers selectively from the frequencies that are used by the basechannelllG and the tour communicating channels C C C and C If the basechannel frequency is live thousand and the tour communicating channelfrequencies are ten thousand, fifteen sand and twent vdi've thousandrespectively, as has been assumed. the trequencj' to which the callingchannel. is adjusted may be thirty thousand. The calling channeltrequeue is illustrated as being supplied to the modulator MC byaseparate oscillator ()(l; but obviously, if desired, the callingcarrier, frequency of thirty thousand may be one oi the harmonics of thebase tre- 'iplied to the modulator MC by 'a cor esponuingly tunedbran-eh ot' the distributing circuit ti',

-18. This alternative arrangement dia' granunatically indicated in Fig.tl where the tuned circuit or filter CF is selective ot' the calling am.frequency and serves to derive the harmonic of this trequenc from thedistributing circuit &7, l8 and to apply it. to

" is connected with} the input circuit of the,

masco the modulator MC. The rest of the circuit is as shown in Figs. 1and 3, the oscillator HOG being, otcourse, omitted when the arrangementof Fig."6"is*used.

The modulation of the calling carrier Wave is efl'ected'by lowerfrequency currents produced by the oscillator O and the harmonicproducer HPL. The oscillator O as in the case of oscillators OP and DCis of the wellelgnown type having a feed-back connection from its outputcircuit to its input circuit byivay' of the inductive coupling 121. 'Theoutput circuit of t he oscillator harmonic producer tubeHPL. The mag-"nitude of the oscillations supplied is such as to merload the tube HPL,thus "causing the tube to distort the sinusoidal wave of the oscillatorQ and produce, an its. output circuit non-sinusoidal currents that arethe equivalentofthe'base frequency current of the oscillator togetherwith theharmonics of the base frequency. This series of. currents is supplied way of transformer 122 to the disa The various of themodulatingcircuits MB, MC and MD.

From the otentiometers the oscillations: of the selected frequencies areconducted to the input circuits of the amplifiers A A and A The outputcircuits of these am plifiers are connected in tuned branches of themodulating circuit M, the tuning of the branches being such as to permitthe passage to the modulating circuit of the frequency of thecorresponding modulating branch and to exclude irorn thebranch 1nquestion the frequencies of the other modulating branches that i'eedintothe modulating circuit'M.

' The modulating circuit M is connected with terminals of a transformer126, the other terminals of which are connected With the input oircuitof the modulator MC.

The modulating. branches MB, MC and are normally;inoperative and aremade operative under theycontroltof thea'ssoeiated I relays 8 and Urespectivelxthese relays actingiu hen operatedzto apply current from hefilament and p-late batteries F and P tohenergige the. apparatus of thecorrespond ing branches. The relays 7, 8 and 9 in turn are controlledrespectively by the keys KB,

KC and KD, Which are the station selecting keys of the organization. 0and harmonic producer-HPL are also normally inert, and are energizedthrough the energization of relay 69, which operates. tdi eonnect thefilament and plate batteries g input circuit of an amplifier A tothearms-.

The oscillator,

l The oscillator 0 may be arranged to produce oscillations of anysuitable fre quency as, for instance, 200 cycles. As a result of theactiouof-theharinonic producer HPL there is transmitted to thedistributing circuit 93, 99the base frequency 200 of the modulator 0together with the harmonic frequencies thereof, If three modulatingpaths MB, MCand MD are employed, these" paths'have their constantsadjusted to divert from the distributing circuit, amplify and pass tothe 'inodulating circuit M the three harmonic frequencies lOO, 6.09 and800. 1i additional modulating branches are to be used, they maybeadjusted to employ the base, frequency 200 and other higher harmonicfrequencies.

,As will subsequently be explained, each of the toll stations is adaptedto have the receiving branch oi its'callin'g channel selec tivelyresponsive to a particular one of these transmitting branch of thecalling channel at each station is arranged to transmit the callingccarrier frequency and as many lower frequency modulations thereofastliere are distanttoll 'stationsto be selectively controlled or"signalled. The transmitting branch of the calling channel at each tollSllitlJlOIrlS of course arranged to modulate the calling channelfrequency only with the lower frequencies to which the distant toll stations responsive, and not with the lowerfrequency to which theparticular toll statii'in itself is responsive. Thus the par-- ticularstation illustrated inthe drawing is assumed. to be responsive to the200 1nodulat-, ing frequency, and consequently the B10631- latingbranches are so arranged as not to apply'tlie 200 frequency to themodulating.

path M,zbut to apply the three harmonic frequencies 400, 600 and 800.Atthe toll eta-'- tion responsive to the lOlliirequehoy the modulatingbranches are arranged to em ploy the 200, 600 and'SOO frequencies; andsimilarly at the other stations the particular modulating frequency towhich that station lower modulating frequencies; shjthat the 'r ice isadapted to respond is not arranged to be applied to thetransmittingbranch.-

At each of thetoll stations thereis a con necticn from the receivingpath Rhy way of a band filter DFC anda shieldedtrausformer 12710 theinput circuit of a detector A IX). The BEHdiiltcr DFC is designed to 7divert into the receiving branch of the calling channel C the loand ffrequencies in eluding the calling channel 'frequeney, in thisinstune'eassuined to he thirty thousand, its modulations extending upand down from this frequency through a range inc determined by themodulating frequencies derived fronithe'moduleting path The outputcircuit of the "detector i3 5} is connectcdthrough a transformer $28:Wl?ll H p mes-mite 5 calling channel controlling keys and cir-- cuitsare held ineffective for a of time.

further period The lighting of the assigning lamp notifies the operatoratthe calling station of the selectedchannelis energized and the characterlsti'c carrier frequency is applied. at the calling station. To theapplication of the carriertfrequency at the calling station thereceiving apparatus'of the selected channel at the called station.responds and causes ,-the, lighting of "the corresponding callingstation, thus causing the supervisory lamp lampassociated'with thespring jack terminal of the selected channel at the called station. Atthe same time the carrier.fre-

quency isi withdrawn' from the transmitting branch of the selectedchannel atthe called "in the calling operators cord circuitto light toindicate to that operator that the operator atthe called station hasbeen signalled but has not yet responded. i When the oper lampassociatedtwith the operators connect ator at the called stationrespondsby plugging into the s ring qjacki-of the selected channelindicate by the lighted calling lamp,

the calling lamp and the calling pilot lamp;

at the called station and the supervisory ing cord circuit at thecalling station are extinguishedfandat the same time the chan- 1 nelselecting switch at the called station -is released and returned tonormal. 'When v gthe connectingc-ircuit plugs areinserted in. the springjacks. of the selected channel at the calling and called stations, thekey or,

guard pilot lamps at the-respective stations are extinguished and thecalling channel controlling circuits at these stations areagainmadeavailahle for use. The extension of the selected'channelto-local lines at the callingand called stations is effected through themedium of the conndcting cord circuits atthese stations, andthe;withdrawa1 of-the connecting circuit plugilfrom the channel springjack at either 11 station causesthe lighting vof the corr spo'ndingsupervisory lamp in thecord cir uita't the other station. Ifthe channelselecting switch at the v calledstation fails to select a channel, the

calling carrier frequency is automatically withdrawnat thecallingstation, and the 'chan'netselectin switch at the called stad vconnection of. the plate battery P and the The operation of the systemof the inven I 'cuits'yof thereceiving branch RC ot the tion isautomatically released.

tion now be described in detail. When an operator atone of the tollpoints 01''. stav .maike the ireceiving branch of the calling "tollstation tions has occasion to utilize thetoll line forestahlish'ingconnection with .anothertoll sta Ition, she firstvpressesthe particular one of the three keysKB, KC- or KD correspondingto the distant station that is to be called.

is locked '50 is also operated.

If it is assumed that the distant station through whichthe connection isto be estali -l1shed .is the station D, then the operator ymomentarilydepresses the key KD. On

the assumption that mother toll station is at the time calling, thiscloses a circuitthat of relay 50, 'upper'armature and back con; tact ofrelay 52, 'upperarmature and back contact ofrelayc 51, innerjcontacts oftimer 132, conductor 34, winding of relay 9, mo mentarily closedcontacts-of key KD,conduc extendsfrom battery through the winding tor33, and bacjk contact and armature of relay 58 to ground. The effect ofthe closure of this circuit is. to energize relay 9, which thereuponlocks up toground byway of its upper armature *andfro'nt contactii If itpressed some other call over the toll line at theincluded contacts ofmay 58, relay 51 or relay 52 as will. hereinafter he de scribed.But-"under. the present assumption thatthe callingcircuit is free foruse, the circuit tracedahovejis closed, the relay 9 as described," andthe relay "The relay -50 in operating closes at its contacts a anenergizing ci'rcuit for relay 57; and that relay inoperating closes atits had 'happenedthat'at the time the ,keywas inner frontcont'act and;armature an energizing circuitfor the key p'iiot lamp 179. Theillumination of thislamp persists until the apparatus at the' calledtoll station has selected a channel for useandldentified the" selectedchannel by causing the lighting of the associatedassignmglamp at thecall 'ing tollboard, as willhedescrihed, Belay in operating also closesa circuit that extends from, battery through the outer armature andfront contact of relay 57, 0021- doctor 49, and thence to ground in Ithe conductor 84 and the left-hand 18 associated withachannel that is attime in useat the calling toll station, This isfor the purpose ofpreventing any alter. ation in the condition of use "of a channelduring' the testing operation that is about to take place.

- Relay '50 in operating also, at its contact.

6, applies grou'nd 'to conductor 37 that has parallelfbranches frombattery extending through thewindingsoffrelay 68 and 78.

mo ulating frequency to ,whiclith'e calling tollstation is normallyresponsive, and thus way of locking contact c and winding of: any I ofthe relays the me i

to prevent any possible operation of the associated relays 66 and 67'while this particular toll station is calling. At the same,

' of the calling channel C. The closure of the filament and platebattery circuits makes operative the oscillator 06 and the modulator MCof the transmitting branch TC, and as a result the calling carrierfrequency is applied to the toll line.

The modulating frequency determined by the pressing of the key KD is notimmediately applied to the calling carrier wave, but awaits theoperation of the timer or slowacting relay The energizing circuit ofthis relay is closed at contacts (3 upon the operation of the relay 50;and when, an interval after the operation of the relay 50 deteru'iinedby the time required for timer 55 to draw in its plunger, the innercontacts of timer 55 are closed, the energizing circuit is completed forthe operation of the relay 69. This relay in operating connects theassociated filament and plate batteries with the circuits of theoscillator O and the harmonic producer HPL, which thereupon operate togenerate the pre-determined base modulating frequency and its harmonics.These modulating frequencies, as has been described, are selectivelyseparated out by the dill'erent appropriately tuned circuits -an d areapplied by way of the associated inductive couplings 95, 96 and 97 tothe modulating branches MB, MC and MD of the transmitting branch TC ofthe calling channel 'C. The only one of the modulating branches that isat the time energized is, under the present assumption, the branch MDcorresponding with the key Kl) that has been pressed. Therefore themodulating frequency which this-branch is adapted to transmit is appliedto the input circuit of the modulator MC associated with the callingchannel oscillator ()0. As a result the calliug carrier wave that isapplied to the toll line is modulated by the particular harmonicfrequency of the oscillator O that is adapted to be transmitted throughthe modulating branch MD. The effect of the application to the toll lineof thecalling carrier wave carrying this particular frequency modulationwill be described in detail hereinafter.

When relay 50 operates'as a result of,

pressing one of the station selecting keys, it applies battery at itscontacts e, f, g and oi; to branches 83 extending to ground through thewindings of each of the relays 1719f the four communicating Schannels.The relays 17 of all of the communicating channels are thus heldoperated during the period of energization' of the relay 50, and act toenergize the receiving brain-hes RC, RC RC and RC of the 26. etc., inoperating close energizing cir-' cuits for the associated relays 10, 20,etc., that extend from battery by way of the right-hand windings ofthese relays. At

the same time that the relay 50 causes the energization of the relays 17of all the channels to cause the response of the relays 10, 20, etc., ofthe channels that are in use, it also applies at its contact d a groundto conductor 89 which has branches to the lefthand .armatures of therelays 10, 20, etc., of the various channels. This causes alockingcircuit to be completed from battery through the left-hand winding ofsuch of the relays 10, 20, etc., as may have become energized as aresult of being associated with the channels that are in use, so thatthese relays arepreventedfrom changing their operated conditionduring'the succeeding test and as- 4 An interval after the operation andlocksignme'nt of a selected channel.

ing up of the relays l0, 20, 'etc.,'assoeiated with busy channels, thetimer. 55 completes the inward movement of itsplunger and applies batterY to the conductor 39... This conductor has ranches to theouterright-hand lays 14,524, et which is associated with a relay 10;20,etc., that has not been operated,

has its energizing circuit closed as soon as the timer 55 closes itsinner contact; and such of the relays 14, 24 etc., that are thus enerized are locked up byway of the associated rent back contact of each ofthe relays 10, 20, etc.,

contact and armature, resistances 14, 24'.

etc., and outer lower back contact and annature of associated relays 15,25, etc., to battery. The channels which atthetime are idle andavailable for selection are thus marked or identified by the fact thattheir associated relays 14, 24, etc., are operated and locked up; theenergizing circuits of the corresponding relays. associated with thebusy channels being held open at this time- 'by the energized conditionof the associated relays 10, 20, etc.

Turning now to the operations that take place at the toll pointsorstations other than the one which is calling, the calling frequencyoscillator ()C of calling branch Qmp''h he pressing of key KD, causes.tl1g /"i"es onse of the relays 66 ass he application of toll stationsoi-the line.'

- lilacs sociat edj n'ith the receiving branches RC 58in operating opensatits back'contactand relay 57.

. op erat-ingalso appliesbattery to coiidnctor 1 49, which. conductor,

scrihed,has branches 84 to lockingcontacts armature the path by way ofconductor 33 over which thefljinit'ial energization of the modulatingbranch relays 7, 8 and 9 is ef- .fected V I KB, I C*fan(l KDare'depriredof control so that the station selecting Keys over these relays and overthe associated re? clay in the path common to the branches throughrelays? -8 and 9. \t :the same time re'layi58 in operating closes at itsarmature and front contact the-energizing circuitof ,Itelay' 5?" inoperating appl es bat :tery t0"conductor,-3 5 to illuminate the key.pilot l'amp 7 Oonsequentlythe instant one ofthe station selectingkeysis pressed atany,

of the toll points, the "station selecting keys at all of theotherltoll"points'are madeineffective, and the associated keypilot.lanip 79 is lighted to warn theoperat'orsat these sari-1; yous otherpoints that thecalling apparatus isjunavailahle fortheirzown use. Eachjot io' therelays 57 attheseIother toll. points :in

as hereinhefore de- "of each of the relays 18'of all ofthe'chann'els andC of the associated station.

.herefore the pressing of any of the sta- -tion selecting keys at acalling toll station has the effect of causingthe continued ener- Iotthe relays -1-8- associate gization, tlin'ingthe calling ogeration ofall with chain nels that are inuseateach of the other toll stations.

The. result'lis to keep all of ,the channels in the condition of usethatthey .were inat theinstiintthe station selecting key at the callingstation was pressed, and;

to maintain the'ur in this condition until the end of thechaniielselecting and assignv energize the oscillator Q ing operation. I

As has previoi1sly"heen' described, the timer at the call'i-ng stationoperates upon. the-completion ofthe inward:movement of its plunger toenergize 1"elz'1y..69fa1ul thus and the harmonic producer HPL "togenerate the. base and harmonic frequencies, one of which, in thepresent instance that controlled by the relay 9,; isapplicd to modulate.the calling carrier wave. At theparticular'toll station that is beingcalled the modulating frequency 'is separated out and passes. by way ofthe filter. LPG; tuned to this particular fre qnency to the associatedrectifier LRO, and is there rectified to cause the response of the 1associated relay 67:] .The filters LPO at the other toll stations, {onthe line are tuned to prevent'the pass hf this particularfreconductolrfii to keepihe station selecting quenc y and thereforetheir relays 67 are not operated.

of stepping magnet of channel selecting switchSj, armature and backcontact of this magnehlwinding of slow-release relay 54, right handfront contact and arnmtnre of rc- .lay. 53, and switch arm SK auditsnormal or resting contact to ground. Magnet 60 in operating interruptsat its armatureand back contact this energizing circnit, andthe armatureof magnet 60 in falling hack acts through the medium of the stepping paW128 andratchet Wheel 63 to cause all of the switch arms to take onestep. As soon as the At the toll station that isheing a called, therelny67 in operating acts through "the medium of relay 53;} to close arircuitthat extends from battery through the winding armature ofrelayh't" formaintaining (luring the operation of-switch S the illumination of theselect ng key pilot lamp 79", disconnects the energizing winding ofrelay '50 from k'eysKB, KC and KD from exercising control, over therelays 5,0,, 7, 8 and 9 (ifthe callingci ciiitaslong as the switch -S'is off of no1 n1al;qand the closure of the circuit throughsthe callingpilot lamp 2'? lights that 190 lan pto indicate to the operator at thecalled toll station that a call for her station is coining. j: r J liponthe ,first step taken by switch S, a

circuit is closed that extends from hatterv 9 through fnorlnally closedcontacts 91 sociated' with tlileholtling pawl of the switch S, switchiii-ms", conductor 83. and-whirling lof relay lT ol' the lirstcl'nmnel(,l to ground.

If the first .coimnnnicating channel C hap- 1m pens to (he in uselocally at the tlll'lt the relayv l7 is alrca'dy energized by a branchenergizing circuit closcd'by' the insertion of aplug in the spring jack1 of the channel, as will; hereinafter he described. 17 or the channelis not already energized, it will. be put in its energized condition bythe advance" of the switch arm-S to the cor-v responclingcontact point,and in becoming It the relay energized it connects the filament andplate l batteries F and P with the associated apparatus o'tthechannel,thus making the receiving branch-RC of the'channel responsive to itsparticular carrier wave frequency.

Consequently ,if the particular channel coriresponding to 'thiscarrfer',wave frequency is in'use atany of its toll station branches,

the relay: 19 associated with the receiving branch of the'channel C atthe called at];

tion is energized and closes the energiiaing circuit of relay 16 by wayof conductor 81,

I the-relay 16 thereupon causing the energizaing channel are notoperated, and the ener:

as long as the switch arms 'arc in engagement with contacts as-'sociated with channels that are in use either locally or at distantstations. lVhen the switch arms encounter contacts associated with achannel that is not in use, the relay 19 ot the particularchannel testedisnot operated when it is made responsive by theoperation of theassociated relay 17, As a result the associated relays that correspondto relays 16 and 10 of the first communicattnzin circuit of'the ste inma 'net 60 v D h b terminating upon switch arm S is opened and remainsopen. When this condition comes about, the continuing interruption ofthe stepping circuit permits the slow release relay 54, which may be ofany desired slow releasing type, to release its armature and close acircuit that extends from batterythrough contacts 91 associated with theholding pawl 'of switch S, back contact and armature of release magnet62, armature and back contact of slow release relay 54, switch arm S andthe contact upon which it is at the time resting, the conductor of thecommunicating channel under test which corresponds wi'th conductor 82 ofchannel 0, and the winding of the relay 18 of this channel to ground.The consequent operation of relay 18 of the tested channel connects theassociated filament and plate batteries with the transmitting apparatusof that channel to energize the same and cause the applia 'ation to thetoll line filhe corresponding carrier wave fremicncy.

Returning now to a consideration of the operations at the calling tollstation, the ap plication at the called station of the particularcarrier ave frequency'of the selected channel, as described above,causes the response of the relay 19 associated with the receiving branchof this channel at tl calling station. Assuming that it is channel Cthat has been found idle and available, the operation of the relay 19 ofthis channel causes the operation of the relay 16, which in turn causesthe operation of the associated relay 10. a It will be remembered thatonly, each of the relays 10, 20, etc., at the calling station haveremained unenergizcd as were associated with idle or available channelsat the time that tit e operator at the calling toll station pressed thestation selecting key,

and that the only ones of the relays 14, 24, etc., that were operatedand locked up by the operation of the timer. 55 were thoseiwhose,associated relays 10,, 9.0, etc., had remamed back. Therefore when therelay 10 of one of 'these channels, in this case channel G is operatedas a result of the application of i the corresponding carrier frequencyat the" called station, a' circuit is closed that extends from batterythrough the outer lower armature and back contact of relay c l5, ;re-

sislauce l tfiarinatiire and 'front contact of rclay 14, enterright-hand armature and front contact of relay 10, andwinding of relay.13 to ground. Relay 13 in closing its outer armature andffront contactappliesparatus of the channel C energized after the relay 50 releases,which occurs a moment later. Relay 13 1n operating also closes acircuitthat extends from battery by way of the inner armatureand front contactof relay'13, assigning lamp 11, and the lower winding of relay 51 toground. The illumination of the assigning lamp 11 notifies the cahingtoll operator that the channel that has been selected for use'inresponse to herkcall is the channel terminating in spring jac 1.

Relay 51 in operating closes: at its inner front contact and armature apath for maintaining the illumination of the selecting key pilot orguard lamp79; and at its outer armature and back contact opens theen'ergizing circuit including the windings of relays 50 and 9 closed atthetime selecting key KD was pressed. These two relays thereuponrelease. Relay 9 in releasing de-. energizes the associatedmodulating,branch Ml), thus withdrawing from the calling carrier wavethe modulating frequency to which the distant station 1) hasresponded.hclay 50 in releasing opens at its contacts r, f. 1 and It the branchenergizing paths by way ,ol these contacts for, the relays 17: of all ofthe communicating channels C, C, (-J and C". The relay 1? 0i. channel C,however, remains energized byway of the path completed through the outerfront con-' tact and armature of relay 13, as previously explained: andthe relays 17 of such of the other channels as may have been in uselocally at the time the operator pressed the calling key remain underthe control of the branch paths completed from their con ductors 83through cont-acts closed when a plug is inserted in the correspondingchannel spring jack. 1 I

Relay -50 in releasing as a result of the operation of rclayifil 11 )Onthe lighting of the assigning lamp 1], also opens at its contacts (Itlic cncrgizing circuit of relay 57, and

hand 1 Win incense that .relay in releasing opens conductor 4:) topermit the relays l8 0f'tl1e various channels thereafter to be under theexclusive con-' troloftheir energizing paths byway of the associatedconductors 82. Likewise re,--

lay"5.0-at;,its contact d opens conductor 89 to break. lockingvpaths byway of theleftof relays 10, 20, etc, and thus leave relays under controlof their right-ha11dwindings the relay 50 in releasing interruptsconductor 37, thus releasing relay 78 to withdraw the calling carrierfrequency from the toll line,

and alsoreleasingrelay 68 to reapply the associated plate and filamentbatteries and re store the normal responsiveness of the re ccivingbranch RC. otthe calling channel (3. At its'contact a relay :50 opensthe energizing circuit of timer 5.5 {and the timer in releasing itsplunger first disconnects battery from the conductor 39, and then at thelimit of the outward movement v of the plunger v connects the conductor.39 to ground. This application of ground to con i doctor 39'shortcircuits such of the relays 14, etc., as have been ;,locked up as aresult of being associated [with available com:

.inunicating channels and whose associated channels have not been chosenfor usein the selecting operat on; These relays tlierefore release,theirelay lhhowever remain ing locked up by reason of its associatedrelaylO havingbeen operated upon the se-i withdrawn as aresult of theselection of an available communicating channel, as described above, therelays 66;and 58 at all of the distant toll stations are released. Therelay 5.8 at eachof the distant stationsin reieasirig applies ground tothe associated conductor 33 to make the station selecting keys at thattoll point, again operative, and

also breaks the energizing circuit of theassociai'ed relay 5?. Relay57in releasing cxtinguishes thekey guard or pilot lainp T9 andalso'opens conductor 49 to permit the-relays 18 of'the variouscommunicating channels at the" associated toll point to be again solelyunder the control of the energizingpaths by way of their conductors 82.j

stored at the various distant toll stations,

and these stations areagain enabled to use 7 the calling channel cancalling th r stacoil when it is extended by the insertion of tions.

In addition to the iib'ove describedresults due to the withdrawalof thecalling carrier frequency, there is atithe calleclrtoll station aturthcr,resultldue to the withdrawal At its contacts of the particulariuodululiug frequency that has been superposed upon the calling car ricrwave. The Witlulrawul-of this 1nodulilting frequencypermits thelc-energization of relay (i7 oi the re: civing brunch RC, and theconsequent de-energization of relay 53. Relay 53 in releasing closes atits left-hand back contact and armature a short circuit of it's ownwinding that extends to ground by Way of the switch arm 8. The effect chis is to render it impossible for a subsequent energization of relay 67toell'ect the operation of the relay 53 until tl1eswithLSIhas beenrestored to normal and has thus been placed in condition to respondproperly toa subsequent call for thisstation, A further effect of therelease of the rclaybtl at thecalled station is to prepare forsubsequent closure the-circuit that extends] through the right-handarma-- ture and back contact of the relay 53.

Taking up again the sequence of events at the calling toll station, whenthe calling oporator has been informed by the lighting of the assigninglamp 1]. ofthe particular communicating channel selected, she insertsthe plug 72 of the cord circuit 7 0 into the associated spring jack 1.-

jack 1, andthence to ground by way of both windings of the relay 15. Therelay thereupon operates andcloses by Way of its twolower armatures andfront contacts the I energizing paths 82 and 83 of the relays l8 and 17respectively, at the same time open- This-closes a circuit. that extendsfrom batterythrough the cord circuit sulieryisory lamp 76, co-0peratingsleeve contacts of the plug 72 and. spring ing at its outer lower backcontact the energizing circuitof the relays 14 and 13, thus releasingthese relays. As hereinbefore described, the relay 13 has been keptoperated since the selection of this communicating channeltoholdenergized the relay 17,50

that the 'relay 17a's a. result of the insertion of the plug continuesenergized tokeep 'the v receiving branch RC of the selected channelinlits operative condition. The relay l8 bf the transmitting branch TCof this channel however has not heretofore been operated, and this relaynow operates to apply the ClltlI'ilCtEllSllO carrler frequency of theselected channel C to the toll line. Relay 15 in operating also closesat its inner upper frontfcontact and'arxnature a circuit byway [ofconductor 87 for the energization of relay 88., Relay 88 in operatingconnects the ,two way repeating coil 33 of channel C with the artificalline or'network N designed "to match the changed characteristics of thecircuit lying to the other'side of the hybrid the plug 72, p Y

When the characteristlc carrier frequency this causes the energizationof relay16 which in turn causes relay to V operate;

Relay 10in operating closes a circuit that entends from battery throughthe winding of relay 59, upper winding of relay 51, calling lamp 12associated with spring jack 1 of the selected channel, upper armatureand back contact of; relay 15, theswitch contact associated with'theselected channel and switch arm S that is resting upon it, windingofrelease relay 56, right-hand back contact and armature of relay 53,switch arm S and its associated contact corresponding to the; selectedchannel, and inner right-hand armature and front contact of relay 10 toground. As a result relays 59, 51 and 56 are operated, and the callinglam 12 of the selected channel is lighted. Re ay 59 inoperatingextinguishes the calling pilotlgmp 27. Relay 51 in operatingcloses a branch energizing circuit to keep the key guard or pilot lamp79 lighted after the energizingbranch by way of the inner front contactand armature of relay '52 has been opened by'the re; lease of relay 52upon the operation of relay 59. Relay 51 also introduces another breakinto the calling relay energizing circuit by way of conductor 31 tocontinue the inoperativeness of the associated relays after the path hasbeen closed by the release of relay 52. The lighting of calling lamp 12informs the operatorat the called station that the call of which she isfirst notified'by thelightiug of pilot lamp 27' is to be extended overthe channel terminating in spring jack from the moment of selection.

18 releases it withdraws the characteristic 1. The operation of relay 56operates the carrier frequency of channel C from the toll line and theeffect of this at the'calling station is to release relay 19 ofthe'receiving branch RC and the associated relay 16. lVhen rcla y 16 atthe calling station is thus released it short circuits the lower half ofthe winding of sleeve relay 15. The resistances of thewindings'of relay15 are so proportioned with rspect to the operating char-.

acteristics of the cord eircult supe visory lamp 76 that whenbothwindings are included in series in the circuit the lamp does not receiveenough current for its illumination, but. when the lower winding isshort circuited the current is sufficiently increased tohght the lamp.As a result therefore of the withdrawal of the carrier frequency ofchannel C at the calledstatiun, the cord 7 circuit lamp 76 at thecalling operators po sition is lighted to indicate to the operator thatthe calling lamp at the called station plug 72 andspringjack 1', and toground by way of both windings of the sleeve relay 15, the normal shortcircuit of the lower winding being at this time opened as a result ofthe operation of relay 16 due to the application of the carrierfrequency of'this channel at the calling station. Therefore when theoperator at the called station inserts plug 72 into spring jack 1 thesupervisory lamp76 remains dark, indicating .to her that thecallingoperator has established connection'with the correspondingchannel.

v When relay 15 at the called station operates as a result of theoperators answering the call, as described above, it breaks at its upperback contact and armature the previously traced circuit includingwindings of relay 59, Til-and 56 and the calling lamp 12. As a resultthe calling lamp 12 is extinguished, the circuit by way of conductor 34:1 01i the energization of relays 50, 7, 8 aud t) is again madeeffect-ire and the key guard lamp 7t) extinguished by the release ofrelay 51, and release relay 56 is de=energized to open the circuit ofreleaseniagnct 62. When the magnet 62 releases the engagement betweenthe end of the arm 64: associated with the right-hand armature and theholding pawl (2). causes the lifting of the pawl to permit the releaseand restoration to normal of the switch arms S S S, S and S At the sametime the mechanical relation that exists between the holding pawl 61 andcontacts 91 causes the opening of these contacts during the release outthe switch, so that switch arms S and S are disconnected from batteryduring the return movement of the switch, The return of switch arm tonormal opens the short circuit of the winding of relay 53, so that therelay is again responsive torelay 6'1 in the receiying branch RC of thecalling channel C. This completes the restorationv to normal of thestation calling and channel selectingapparatus at all of the toll pointsor stations, and the conversation is now-free to proceed between thecalling stationand the called station over the communicating channelthat has been selected. I

The operators at, bothttoll points may now complete the connectionwith'the subsoribers station. is calling and the suhseribera I 1 incensestation that'is being called. In the present instiaince this isillustrated as being-accomplished by'means of connecting cord:- cir.

cults provided with the usual plugs 71' and 72 with their associatedconnecting cords and intermediate apparatus, operators set 92, listening-key 73', ringing key 74,- and superv'isory lamps75 and 76. The cordcircuit maybe connected with any client a number i of lines extending tosubscribers stations- SS by inserting the plug .71 into- ,the springjack-77 of the line with which connection is to he established. Thesuhscrib'ers lines may be providecl with the usual'cut-oif'andline'relays and line lampsh The apparatus shown inthis connection isInerQIyQiIIustratiVe; as

any desired one of alarge number of wellknown terminal and connecting;arrangements may be employed. r I V y When the conversation between theconnee-ted subscribers is completed; the subscribers disconnect signalsare received in the usual manner upon the supervisory lamps 75 atthetwotoll stations. When either op orator withdraws the'plug 72 from thecoininunicating channel'spring jack in which it has been. inserted,the'sle evc relay 1!) is released."' Thi's 'leleases relays 17 and 18 ofthe channel, tlmsiwithdrawing the character-' istic carrier frequencyfrom the channel at V [the endwhere the plug is removed, and alsorendering thcreceivingf relay 19 of the channe-l unresponsive.-'Upon'the withdrawal of the carrien frequency at either end otthe channelthe relay 19 at the other end releases thus releasing the associatedrelay 16 and short icircuitint" the lower winding of the correspondingrelay 15. This has the effect of increasing the flow of current in thesleeve circuit'so that theslq'iervisory lamp '76 associated with theplug 72 that is still inserted is lighted. Thus either ope 'ator bytaking down theconncction ca'uscsthe lighting of the cord circuitsupervisory lamp 76 at theo'therfend of the ehanneh an'd .gircs thedisconnect signal to the distantoperator. In the foregoing descriptionof the opera- 'iion; it has been assumed that when the switch S teststhe channels as aresult of the 'ressin of the .station-selectin thex" atthe- ?f Y b calling station, one of these channels is seg. lectable.

for an y reason the arms oi the switch S pass over their contactswithout selecting, means are provided :tor autcnnatically terminatingthe calling condition on the toll line and-for restoring the selectingswitchto normal. These results are accomplished "as follows:

Vs hen the slow acting device or timer operates a predetermned intervalafter the pressing of the key at the calling station and" i applies thestation-selecting modulating frequency and starts the movement of theselecting swi'tch S-atthecalled station, it also If it should, happenthat atJthiS- time all ot-the channels are in use. or that other slowacting'relay device or timer-132.

-T he timer 132 thereupon starts to draw in its plunger. o The intervalre ul'red for the complete inward movement 0 theplunger ot' timer 132isso regulatedas to' be someto test allot the channels to which it hasaccess. Consequently if theswitch S does not nialiea selection, therelay 50 at the callingstation is not'released and therefore the timer55 at the calling station isalso not released. The timer 55 consequentlyholds the energizing circuit of the timer 132 closedv long enough topermit the full inward movement of the plunger of the tinier 132. henthisoccnrs the inner contacts of'tinier 132 are opened, and theenergizing circuit including=ther elay 50 and the particular one Closesa circuit for the energization ofanwhat greater than-the intervalrequired-tor fthe. selecting switch S at the called station of therelays/7, 8 and Qthat has been-op gerated is opened, thus releasingthese relays. This effects the restoration of the calling stationapparatus to normal, in the manner previously described, thuswithdrawing'the calling 'arrierwaye and its modulating frequency. Uponthewvitliclrawal of the call i g carricrwayejand its 'inodnlatingfrequency'the relay53 at the called station is released. The switch S;having failed to contact points associated with the various "channelsand has le'ft'its, switch arms rest- I ing upon thefirst contacts beyondthe chan- -nel contacts. Therefore, when relayf53 at the called stationreleases, a circuit isicl'o'sed that extends from battery throughcontacts 91 associated withthe holding pawl. of switch S, contacts 98associated with the releasing lever 6% of release magnet-62, the

nialge z 1"'selection,'has adyancedbeyond the. u

contact upon which switcharm Sis resting and that switch arm, winding'ofrelease leases and brings about the release of the unagnet The magnet(32 in releasing lifts the holding ,pawllil and permits the arms of theswitch S to return to normal. Thus.-

'the calling condition upon theline is withdrawn and theswitclixSrestored to normal upon the. tailureot the switch to. select as?channel. I

opening of'thcsc contacts breaks the energiz in'g cirouitlot relay 56,which thereupon re- When the, energizingcircuit at the calling stationfor. the relay 50'and the particular one of the relays 7, 8 and-9 thathasbeen operatedis broken by the -complete inward? Stroke of the timer132 as a result of the switch S at the called station failing to findand energize an idle channel, as described,

thecircuit for relay 57, and consequently the circuit for lamp 79, arealso broken so that the calling operator upon observing that the1ampQ79- is extinguished is apprised of.

the fact that no idle channel is available.

The invention is not to be construed as limited to the specific detailsof the foregoing description or of the drawing, but its ,sco e isdefined inthe appended claims.

, hat is claimed is: 1. In a carrier .wave communicatingsystem,.the,comblnavlon w1th a line and three or more stations connectedthereto, of frequency responsive means operatively associated with saidhne at each station and selectively responsive to a particularfretransmitted at any instant by said line for producing and applying-tosaid line said waves to which the various other stations are adapted torespond.

2. In a carrier wave communicating system, the combination with a linehaving a plurality of stations connected thereto, of

'wave responsive means at each station,

means at each'st'ation controlled by said wave responsive means forroducing and applying to said line a plurality of different frequencywaves, and means at each of a plurality of said stations selectivelyresponsive to aparticular one of said waves.

3. In a carrier wave communicating sys tem, the combination with a linehaving a plurality of stations connected thereto, of means at eachstation'for producing and applying to said line a plurality of difierentfrequency waves, means at each other station selectively responsive to aparticular one of said waves, signals at the stations at which the wavesare being produced and atthe station selectively responsive to thecorresponding wave, and means for actuating said signals responsive tothe means at the station selectively responsive to the correspondingwave when said means is actuated.

4. In a high frequency communicating system, the combination 'with aplurality of.

high frequency-stations, of means at each stationforlproducing andtransmitting a plurality ofdifiering high frequency waves, means at eachstation selectivel responsive to a particularone of said higi frequencywaves, a high frequency communicating channel having terminals at saidstation, and means operated by the response of the station to itscorresponding wave for preparing said channel ;;for communication.

5. In a carrier wave communicating system, the combination with a lineand a plurality of stations connected therewith, of a calling channelassociated with said line and having a plurality of branches at eachstation, meansfor energizin said calling channel and' any one of theranches thereof at any 0 said stations, and apparatus at each.

station adapted to be selected and operated at a particular one of saidstations in ac cordance with the particular calling channel branch thatis energized.

' 7. In a carrier wave communicating system, the combination with a lineand a phi-- rality of stations connected therewith, of a calling channelassociated with said line and having a. plurality of frequency selectivebranches at each of said stations, means for energizing said callingchannel and any of said branches at any oi said stations, apparatus ateach station selectively operated in accordance with the particularcalling channel branch that is energized, a plurality of frequencyselective communicating chain nels connected with said line andavailable for use at each of said stations, and apparatus at thecallingstation and the selected station. for establishing communication overone of said communicating channels.

8.1n a high frequency communicating system, the combination with aplurality oi high frequency stations, ,of means at each station forproducing and utilizing a plurality of high frequency waves, meanscontrolled at a calling one of said stations for utilizing one of saidwaves to select another one of said stations, means at the selectedstation for selecting another of said high frequency waves, and meansfor establishing communicating relation between said calling station andsaid selected station through the medium of said selected high frequencywave.-

9. In a high frequency connnnnicating system, the combination with aplurality of stations each having means for producing and utilizing aplurality of high frequenr station for selecting another of said highfrequency waves for communicatingpurposes, means for identifying theselected 'wave at said calling station, and means at the calling andselected stations for establishing communicating relations therebeinn

